1. Field of the Invention
The invention relates to a ball screw deflector.
2. Description of the Related Art
Conventionally, there has been known an electric power steering system (hereinafter, referred to as “EPS”) that assists a driver in performing a steering operation by applying driving power generated by a motor to a steering mechanism of a vehicle. In an EPS, a rack-and-pinion mechanism is usually adopted. The rack-and-pinion mechanism converts the rotational motion of a pinion caused by the operation of a steering wheel into a linear motion of a linear-motion shaft having a rack that is engaged with the pinion. The orientation of wheels is changed by the linear motion of the linear-motion shaft. In a rack assist-type EPS, a linear motion shaft is provided with a ball screw mechanism that converts the rotational motion of a motor into a linear motion of the linear-motion shaft.
The ball screw mechanism is formed by screwing a ball nut to a ball screw portion formed on the outer periphery of the linear-motion shaft with a plurality of balls interposed therebetween. In the EPS, the ball nut rotates, and therefore the ball nut is preferably compact in size in the radial direction in view of rotational balance, for example. Thus, the structure in which the ball nut is provided with a deflector (circulation member) that causes the balls in the ball screw portion to roll and circulate is widely adopted.
Japanese Utility Model Registration No. 3165152 describes a deflector for a common ball screw mechanism. The deflector is formed so as to be elongated in the axial direction of a linear-motion shaft. Thus, the structure in which the deflector is formed of a plurality of separate pieces is proposed in order to insure accuracy.
For example, as illustrated in FIG. 9, at the left end (in FIG. 9) of a deflector piece 100, there is formed a left end face 101 of the deflector piece 100 having an angle corresponding to a lead angle of a ball screw portion. Thus, it is possible to form the deflector piece 100 from a material that is reduced in amount by an amount corresponding to a hatched region 110. A circulation path 105 through which balls located at the ball screw portion are circulated is formed in the deflector piece 100.
The deflector piece 100 is connected to another deflector piece 120 at a mating face 115. The mating face 115 extends in a direction orthogonal to the axial direction of a linear-motion shaft (in the up-down direction in FIG. 9).
However, in this structure, the left end face 101 and the mating face 115 are not parallel to each other. Therefore, it is difficult to measure the overall length of the deflector piece 100 with the use of a vernier caliper, for example. That is, when the deflector piece 100 is held by two gauge heads of the vernier caliper, the positions of the gauge heads are not fixed. This causes variations in measured values, which reduces the reliability of the measured values. Although three-dimensional measurement may be adopted, the three-dimensional measurement is not preferable in terms of measurement time.